An insulating film which is to be a sidewall is formed on the sidewalls of a gate electrode of an MOS transistor, and source and drain regions are formed by implanting an impurity into both sides of the gate electrode. At the end portions of these source and drain regions, however, many crystal defects appear in a silicon substrate. As a method of preventing these crystal defects, Japanese Patent Laid-Open No. H8(1996)-97210 (Patent Document 1) discloses a mode in which the influence of a stress caused by a nitride film formed on the side surface of a gate electrode is suppressed by forming an oxide film on the side surfaces of the gate electrode and between a silicon nitride film to be sidewalls and a substrate therebelow. [Patent Document 1] Japanese Patent Laid-Open No. H8(1996)-97210